Arrangements or metering pumps of the type described above have been known for a long time back and are used frequently in situations where media are to be conveyed in defined doses or flow rates for one purpose or another. This may refer to liquids, e.g. reagent solutions in connection with a chemical analysis process or a larger-scale industrial process which requires continuous supply of starting material at a correctly weighed out flow rate in order to achieve the optimum quality characteristics of a desired product, but situations also occur where gases or outright solid materials in finely divided fluid form are to be fed in volume-controlled portions according to some required working scheme.
As indicated already the field of application for the known metering pumps is highly diversified both with regard to the types of media which are to be metered and the required size of the feed doses and flow rates respectively. The respective types of application have in common, though, that the metering pump used should fulfil at least a certain degree of accuracy of performance, so that the quantity of the particular medium actually fed corresponds as closely as possible to the quantity desired on the occasion. It happens frequently, though, that it is desirable, or necessary, to adapt the quantity of feed to a varying consumption requirement, so that, for example, during a certain period a large quantity of the medium has to be fed, whilst during a subsequent period a smaller quantity may be sufficient. To make possible such an adjustment or change-over between different desired doses or flow rates in a flexible manner, it is necessary consequently for the metering pump used to possess a corresponding flexibility of capacity. On certain known metering pumps of the type described with movable pump piston such a facility to change over between different desired metering volumes or flow rates has been made possible in that the driving arrangement or the motor which is used for driving the pump piston has been provided with some type of controllable power transmission arrangement to allow increase or decrease of the stroke frequency of the pump piston, as required. This type of variable metering pumps or, more precisely, this arrangement controlling the transmission, which is required in order to drive the pump piston at the desired rates, for obvious reasons, is greatly subject to wear and requires frequently recurring inspection and maintenance, especially if the metering pump is to be used in contexts demanding frequent changes between different metering volumes and which consequently put particularly severe loads on sensitive driving components for the pump piston.
Known adustable metering pumps of a type somewhat more reliable in operation and simpler are those which operate at one and the same stroke frequency of the pump piston for the different metering volumes desired and which therefore do not require any supplementary, sensitive driving equipment of the type described just now. In the latter type of known metering pumps the desired variations in metering volumes are achieved instead with the help of stop elements limiting the stroke length of the pump piston which may be arranged either stepwise or freely movable in the pump cylinder or metering chamber. Metering pumps of this type certainly have a simpler and therefore more easily maneuverable design than the metering pumps with changing of the stroke frequency described above, but they present, on the other hand a somewhat less reliable accuracy of performance owing, among other things, to the elements, set for a desired metering volume, being very much subject to the risk of unintended shift of their set position as a result of the pump piston repeatedly striking against them. This risk naturally can be particularly serious where metering pumps are intended to be used for the metering of very small metering volumes required, since each smallest displacement of the element in such a case may give rise to substantial fault deviations from the metering volume desired.
Against this background there is an apparent demand for a metering pump which is simple in its design and maneuverability and which requires little maintenance and which nevertheless allows flexible shifting between different metering volumes desired whilst retaining accuracy of performance, also during prolonged usage.